(On four occasions, “come as a thief”, is the point being made.The first, to the church in Sardis, for people to consider during the church age:)
Revelation 3:3 Remember therefore how thou hast received and heard, and hold fast, and repent. If therefore thou shalt not watch, I will come on thee as a thief, and thou shalt not know what hour I will come upon thee.
(Either to those in the church in Sardis in John’s day, or to anyone reading this during the church age,chastisement can come suddenly, so it is far better to repent before that happens.)
(The next, to the church in Thessalonica, also for people to consider during the church age:)
I Thessalonians 5:2 For yourselves know perfectly that the day of the Lord so cometh as a thief in the night.4 But ye, brethren, are not in darkness, that that day should overtake you as a thief.
(The blessed hope, and thus most likely the beginning of the tribulation, will come suddenly, no one will know the day or hour.Jesus comes suddenly in the air, and those who are saved, just as suddenly are caught up, and go to heaven with Jesus.For the saved, that is the blessed hope, but for the lost, who are still in darkness, it is a dire moment.The tribulation has come upon them, overtaking them, like a thief suddenly and unexpectedly overtaking them.)
(The next is at the end of the tribulation, right at the time of Armageddon, and the second advent of Jesus:)
Revelation 16:14 For they are the spirits of devils, working miracles, which go forth unto the kings of the earth and of the whole world, to gather them to the battle of that great day of God Almighty.15 Behold, I come as a thief. Blessed is he that watcheth, and keepeth his garments, lest he walk naked, and they see his shame.16 And he gathered them together into a place called in the Hebrew tongue Armageddon.
(The armies of the world gather against Israel to take Jerusalem.No doubt they are expecting victory from their overwhelming forces arrayed against Israel.To them this is just another war, one that they have been thinking about for a long time, one they expect to win.What they don’t expect is for Jesus to suddenly return from heaven, and to destroy their armies, like grapes in a winepress.)
(However, the principle applies throughout the church age:When Jesus suddenly does something, we don't want to be caught out of fellowship, and to be ashamed.Like the one time we decide not to come to church, is the day that our family member, friend or coworker,decides to come to church, and that day they get saved, and we’re not even there, and we look like a hypocrite.)
(The next may be at the end of the thousand year kingdom, when the devil is loosed from the bottomless pit:)
II Peter 3:10 But the day of the Lord will come as a thief in the night; in the which the heavens shall pass away with a great noise, and the elements shall melt with fervent heat, the earth also and the works that are therein shall be burned up.
(Those people who apparently do not trust Jesus, even though they live in the time of the kingdom, with Jesus on the earth,are swept up into the rebellion, with many others in the same situation, are suddenly caught by surprise,instead of a fight, they are suddenly burned up, and find themselves at the great white throne,and then judged and cast into the lake of fire.)
(The point of the term, “come as a thief”, in these four occasions, implies the suddenness,and that those not prepared, are caught off guard, suddenly, for them it is unexpected.)